Polarized media and other active field air cleaners are typically designed so that two framed screens are hinged and latched together. The frames are typically made of a rigid material such as a rolled or extruded aluminum rail. Hinges and latches are attached to these.
There are issues, however, with this approach to air cleaners that slide into a track. First, the tracks themselves are made of metal and are not consistently fabricated unit to unit. Therefore to ensure that the frames will slide into the tracks, the frames are often undersized and will not seal tightly. This results in blow-by of air around the air cleaner that is not cleaned, degrading system performance. Second, this is exacerbated by the fact that if the latches and hinges protrude from the surface of the frame, the frame must be undersized to accommodate that dimension, ie the frame will have to be even smaller—allowing for more blow-by.
Third, the tracks themselves are often attached with screw or rivets and the latches and hinges of the air cleaner typically protrude from the surface of the frame and can catch on the rivets and screws, making service and installation difficult.
Fourth, in many cases, the construction of the track and overall duct system is such that there is ductwork or other sheet metal on either side of the track opening. Here, the filter can slide into the track and be flush or recessed relative to the surrounding structures. Therefore, once the filter is installed in the track, there is no surface by which one can easily get a hold on the air cleaner when it needs to be pulled out and removed for service and media replacement. This makes it harder to remove an air cleaner from the track and can lead to damage of the air cleaner when tools such as pliers or screwdrivers are used to remove the filter.